1. Field of the Invention
The present invention relates to a method and apparatus for changing the shape of the cornea of an eye to correct refractive disorders and, more particularly, to such a method and apparatus which selectively injures or kills keratocytes and avoids shrinking the collagen in the cornea.
2. The Prior Art
Myopia and hyperopia are conditions wherein the light entering a person's eye is not properly focused on the retina. Astigmatism is also the result of improper focus of light onto the retina. Light passes through the cornea of the eye to define an image, and it is now common to attempt to correct for refractive disorders of the type mentioned above by changing the shape of the cornea. Various surgical procedures have been employed, or have been suggested, for this purpose. Most of these procedures involve either mechanical penetration of the cornea or the intentional physical destruction of corneal tissue.
One of these procedures, radial keratotomy, is accomplished by making incisions in a radial pattern around the outer periphery of the cornea; this causes the periphery to bulge slightly and the center of the cornea to flatten. Another of these procedures, lameliar refractive keratoplasty, is accomplished by the placement of material on, or in, the cornea to change its shape. For example, a disk of corneal material from a donor is sutured to the anterior (outer) surface of the cornea, or a synthetic material is implanted within the cornea.
Laser photorefractive keratectomy is accomplished by recontouring the anterior layers of the cornea via ablation of portions of those layers. Intrastromal ablation is accomplished by the use of a laser to vaporize selected areas in the center of the cornea, effectively changing the cornea's shape as the outer portions of the cornea relax into the pockets created by the procedure. Thermal keratoplasty achieves a change in corneal curvature by permanently shrinking collagen fibrils in the cornea. U.S. Pat. No. 4,976,709 to Sand teaches an example of this approach of shrinking collagen fibrils.
The prior art in the area of thermal keratoplasty uniformly teaches achievement of the desired effect by employing treatment techniques which result in collagen shrinkage. Examples of such prior art known to the inventors include U.S. Pat. No. 4,381,007 to Doss; Doss, J. D. and Albillar, J. I., "A Technique for the Selective Heating of Corneal Stroma," Contact Lens, Vol. 6, No. 1, p. 13, Jan.-Mar. 1980; Gasset, A. R. and Kaufman, H. E., "Thermokeratoplasty in the Treatment of Keratoconus," Am. J. Ophthalmol. 79, p. 226 (1975); Doss, J. D., Hutson, R. L., and Albillar, J. I., An Electrothermal Technique for the Alteration of Corneal Curvature," Los Alamos Laboratories Informal Report LA-7155-MS, issued February 1988; Mainster, M. A., "Ophthalmic applications of infrared lasers-thermal considerations," Inv. Ophth. & Vis. Sci., Vol. 18, No. 4, p. 414 April 1979; Rowsey, J. J., Gaylor, J. R., Dahlstrom, R. and Doss, J. D., "Los Alamos Keratoplasty Techniques," Cont. & Intraoc. Lens Med. J., Vol. 6, No. 1, p. 1, Jan-Mar. 1980.
All of the surgical procedures identified above have disadvantages associated with them. Many of these disadvantages result either from a mechanical penetration of the cornea or from a permanent weakening of the structure of the cornea. Any penetration of the cornea includes the risk of infection. Certain procedures, particularly radial keratotomy, also pose the risk of microperforation of the cornea. Corneal inserts introduce the risk of rejection by the cornea. Laser photorefractive keratectomy destroys a portion of a non-regenerative layer of the cornea (Bowman's membrane, which serves as the anchoring membrane for the outermost layer of the cornea), as well as destroying corneal stroma.
Even more disadvantages result from the effects of physical destruction or alteration of corneal tissue. The cornea is uniquely structured to transmit light into the eye. The primary structure of the cornea is the stroma, which comprises approximately 90 percent of the cornea's thickness. The stroma is composed of many relatively long and wide lamellae which run in flat layers from limbus to limbus. The lamellae consist of uniform, parallel, fine collagen fibrils, which are regularly spaced within the lamellae and separated by glycoproteins. It is the thinness of the collagen fibrils and the regularity of their direction and spacing that result in the transparency of the cornea. When this uniformity of size or spacing of the fibrils is disrupted, the proper transmission of light directly into the eye is diminished. It will be appreciated that any surgical procedure leading to a diminishment or disruption of light entering the eye creates an undesirable result. Even when destruction of tissue is avoided, as can be accomplished with laser thermal keratoplasty, the shrinkage of collagen fibrils increases their girth, resulting in an undesirable opacity of the treated area. Current human trials applying this procedure have avoided treatment of the center 3 mm of the cornea, presumably because of the resulting opacity.